High strength is required for steel sheets to be generally used for automobile framework components and the like for the purpose of collision safety, fuel consumption reduction due to reduction in car body weight and the like, and excellent press formability is also required in order to work them into the framework components that are complex in shape. Further, material designing on the basis of only tensile strength (TS) has conventionally been executed. However, when collision safety is taken in consideration, material designing on the basis of yield strength (YP) becomes necessary. Accordingly, a high strength steel sheet also excellent in the yield strength (YP) in addition to the tensile strength (TS) and excellent in workability has become demanded.
For this reason, it has been earnestly desired to provide, for example, a high strength steel sheet having a yield strength (YP) of 1,180 MPa or more and a tensile strength (TS) of 1,470 MPa or more and having a bendability (critical bending radius/sheet thickness: R/t) of 2.4 or less (preferably 2.1 or less, and more preferably 1.5 or less).
In consideration of such needs as described above, there have been proposed many high-strength steel sheets improved in bendability, on the basis of various ideas for component designing and structure control. However, there are still few ones in which all of the yield strength, the tensile strength and the bendability satisfy such desired levels as described above, at the present stage.
For example, Patent Document 1 discloses a high tensile strength cold-rolled steel sheet substantially composed of a single-phase microstructure of martensite, and in the steel sheet having a tensile strength of 1,470 MPa or more, a bendability (critical bending radius/sheet thickness: R/t) of 1.5 or less is obtained in a bending test by a three-point bending method. Also, there is only one example in which the tensile strength is 1,470 MPa or more, the yield strength is 1,180 MPa or more and the above-mentioned bendability is 0.75 (see Table 3, No. 8). However, in all of these examples, Ti and Nb are added in order to increase strength, particularly the yield strength, and in the above-mentioned example No. 8, B is further added in addition to these. Accordingly, Ti and Nb are essential for ones described in the document, and it is considered that the bendability is necessarily deteriorated by carbides produced by addition of these elements. It is therefore unlikely to obtain the bendability satisfying the demands described above by a V-block method that is a severer evaluation method.
Patent Document 2 discloses a high tensile strength steel sheet composed of a dual-phase microstructure of 25 to 75% of ferrite in terms of area ratio with the remainder being tempered martensite, and the bendability (critical bending radius/sheet thickness: R/t) of 2.4 or less is obtained by a test method in accordance with JIS Z 2248. However, 25% or more of ferrite as a soft phase is contained, so that a tensile strength of 1,470 MPa or more is not satisfied as shown in the examples thereof, and from the level of the tensile strength, it is presumed that the yield strength will be of course less than 1,180 MPa.
Patent Document 3 partially discloses the high tensile strength steel sheets having a tensile strength of 1,470 MPa or more and satisfying a bendability (critical bending radius/sheet thickness: R/t) of 2.4 or less in a bending test by a U-bending method, targeting a tensile strength of 980 MPa or more in the examples thereof. However, it is unlikely to obtain the bendability satisfying the demands described above by the V-block method that is a severer evaluation method. Moreover, the workability is enhanced by precipitating carbides in large amounts, so that the C solid solution amount is small. Further, the area ratio of soft ferrite is large. Accordingly, the yield strength (YP: indicated as YS in the same document) is in the level of at most 1,100 MPa or less, and does not satisfy the requirement of 1,180 MPa or more.